This invention relates to a photovoltaic (PV) energy conversion device, specifically a thin film n/p or n/i/p heterojunction with graded carrier concentration, using semiconducting compounds from the I-III-VI.sub.2, I-III-VI-VII and I-VI.sub.3 -VII series. It relates further to methods of manufacturing the cells, directed to producing a cell, especially an indium-tin-oxide (ITO)/n-CulnSe.sub.2 /i-Culn.sub.x Se.sub.y I.sub.z /p-CulSe.sub.3 cell, with relatively high efficiency, high stability, low cost, and low toxicity.
Photovoltaic cells offer the prospect of a more benign and renewable source of power than either fossil or nuclear systems. The criteria for a successful photovoltaic device are high energy conversion efficiencies, long-term stability and low cost. The high efficiency solar cells are based on monocrystal absorbers, for example, Si, GaAs, InP, and CdTe, which require high energy, labor, and highly purified materials. The less expensive amorphous Si cell is unstable and less efficient. CulnSe.sub.2 meets the criteria of low cost, high efficiency, stability, and environmental safety. It has thus emerged as the most promising material for terrestrial and space applications. It is a direct band gap semiconductor with a low minority carrier diffusion length, a high absorption coefficient, and a steep absorption edge. These attributes permit fabrication of lightweight, low material-usage thin film cells; its radiation hardness ranks CulnSe.sub.2 ahead of Si and GaAs for space use.
In the prior art, p-CulnSe.sub.2 has been used in thin film Mo/p-CulnSe.sub.2 /n-CdS/ZnO cells (p-CIS) by various workers. U.S. patents to Mickelson et al (1980) U.S. Pat. Nos. 4,335,266, 4,465,575; Kapur et al (1986) U.S. Pat. No. 4,581,108, Ermer et al (1989) U.S. Pat. No. 4,798,660 provide background information on the development of the p-CIS cell. Numerous deposition techniques have been employed, including: (a) selenization of Cu/In layers; (b) evaporation (thermal, electron-beam, sputtering or ion plating); (c) electrodeposition; (d) chemical spraying; (e) screen printing; (f) sintering; and (g) laser annealing. The description of these methods and the resulting films along with references are summarized by Zweibel et al (1989), Chopra and Das (1983). The key processes that have yielded high quality p-CIS films are co-evaporation of Cu, In, and Se, and selenization of electron beam deposited Cu and In layers. Although single junction efficiencies in the range of 14% have been recently reported, the sensitivity of cell performance to several processing steps and safety issues associated with some cell components continue to be of serious concern and have hindered its commercialization. For example:
(i) the toxicity of the CdS window component has been the primary deterrent in its terrestrial use (Cd and its compounds present health, safety and environmental hazards, e.g. Moskowitz et al (1990)); PA1 (ii) the Mo contacts to p-CuInSe.sub.2 affect the mechanical stability (poor adhesion) and the performance of the cell, possibly due to the formation of a layered compound, MoSe.sub.2, at the interface. Mo is also toxic to some degree; PA1 (iii) interdiffusion of Cu and Cd at the heterojunction interface can induce conductivity type inversion in thin films of p-CuInSe.sub.2 which could undermine the cell stability, especially in radiative environments; PA1 (iv) lattice mis-match of 1.2% between CuInSe.sub.2 and CdS increases interface state density, leading to efficiency losses due to the associated recombination.
The n-CuInSe.sub.2 material has not been employed in a thin film photovoltaic cell for two reasons: (a) first, the difficulties encountered in preparing non-resistive n-CuInSe.sub.2 films, e.g. Thornton (1987) indicated that the deposition of In-rich films by reactive sputtering in H.sub.2 Se medium led to In rejection; other workers have noted similar In rejection for the co-evaporation and in the e-beam deposition/selenization methods. Noufi et al (1987) reported the formation of highly compensated and resistive n-type films by co-evaporation. Similarly synthesized films were unsatisfactory even after vacuum annealing at high temperatures in the presence of Cd or In, according to Haneman (1990); and (b) the second reason was the lack of a highly conducting and transparent p-type window, as most wide bandgap materials, e.g. oxides, are n-type.
Nevertheless, high efficiency photoelectrochemical cells with n-CuInSe.sub.2 single crystal were reported by Menezes et al (1983, 1984, 1986) and by Cahen (1984). Our U.S. Pat. No. 4,601,960 (1986) describes the first n-CuInSe.sub.2 photoelectrochemical cell that was stabilized from corroding in the electrolyte by inducing the growth of a new semiconducting interphase from the corrosion products. The n-CuInSe.sub.2 single crystal based solar cell with a CuISe.sub.3 interfacial film was 12.2% efficient and stable. An energy difference between the work functions of CuInSe.sub.2 and CuISe.sub.3 suggested the possibility of a buried p/n heterojunction between the two materials. Although our approach at surface stabilization was reproduced by various other workers, no further evidence of a solid state junction was reported. Thus, no solid state cell has been constructed primarily because (c) large single crystal CuInSe.sub.2 substrates are difficult to fabricate and impractical for scale-up and (d) since CuInSe.sub.2 thin films tend to corrode and peel off the substrate during surface conversion in the electrolyte.
The uncertainties regarding the existence of a solid state junction persisted because of the above mentioned difficulties in verifying the concept of a buried p/n junction. These difficulties also prevented the construction of a practical photoelectrochemical cell. In general, the utility of photoelectrochemical cells is limited by several engineering constraints:
(e) Inflexible cell design;
(f) Susceptibility to leaks and corrosion at the various inherent solid/liquid interfaces;
(g) Uncertain long-term stability of the electrolyte/semiconductor interface in hostile environments; and
(h) Need for expensive bulk (thick) single crystal or polycrystalline CuInSe.sub.2 substrates which are not economical in terms of material and energy usage, and unsuitable for large area applications (Single crystal grain size rarely exceeds 2 cm.sup.2).